To un-install (remove), use the add/remove programs utility found in the Windows control panel.
1. Check that any anti-virus software or fire wall is temporarily disabled.
2. Run the downloaded product Wrapper file (this is a file with the red airplane icon attached to it). If you don't still have it/them you'll need to download it/them again
3. Run the file, and when presented with the purchase screen, ignore everything on it, except for pressing the Reinstall button at the lower right of the screen.
4. When prompted, point to your file and enter the 4 number code (the last 4 numbers from your purchasing credit card, or a 4-digit pass code that you may have been provided with).
5. When the purchase is validated the actual installation file will be extracted (to a location of your choice or the default C:\Flight One Software or C:\DreamFleet folder) and run.
6. After the installation has finished, do not move the Key.
For further details or help, this is the appropriate page at Flight1:
If you have lost your product key, please see this post, which describes how to get a new key.